Information may be recorded by exposing a portion of an optical recording medium to a recording light beam thereby changing the local optical properties of the exposed portion. The simplest such recording medium is a monolayer structure having a light absorptive layer overlying a substrate. Information is recorded by locally melting or ablating the light absorptive layer to change the transmission or reflectivity of the recording medium in the exposed portions.
Spong, in U.S. Pat. No. 4,097,895, issued June 27, 1978 which is entitled MULTILAYER OPTICAL RECORD and which is incorporated herein by reference, disclosed a bilayer optical recording medium which comprises a light reflective layer coated with a light absorptive layer. Bell, in U.S. Pat. No. 4,216,501, issued Aug. 5, 1980, which is entitled OPTICAL ANTI-REFLECTIVE INFORMATION RECORD and which is incorporated herein by reference, disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorptive layers of the bilayer recording medium.
Such recording media are substantially uniform in their structural and optical properties prior to exposure and thus contain no means by which a track can be defined or followed prior to the recording of information. A recording medium having a pregrooved substrate which has been proposed to provide these means, does not have the flexibility for changes of the track arrangement after manufacture. An alternative approach in which a portion of the absorptive layer is thermally removed by a laser beam to form the track has this flexibility but is undesirable since it involves a modification of the absorptive layer itself. It would be desirable to have a medium in which tracks can be defined after the fabrication of the medium without perturbing the absorptive layer.